(eplain.info)Formatting equation references

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Enter node , (file) or (file)node

4.11.1 Formatting equation references
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Both defining an equation label and referring to it should usually
produce output. This output is produced with the '\eqprint' macro, which
takes one argument, the equation number being defined or referred to. By
default, this just produces '(NUMBER)', where NUMBER is the equation
number. To produce the equation number in a different font, or with
different surrounding symbols, or whatever, you can redefine '\eqprint'.
For example, the following definition would print all equation numbers
in italics. (The extra braces define a group, to keep the font change
from affecting surrounding text.)

     \def\eqprint#1{{\it (#1)}}

   In addition to changing the formatting of equation numbers, you might
want to add more structure to the equation number; for example, you
might want to include the chapter number, to get equation numbers like
'(1.2)'. To achieve this, you redefine '\eqconstruct'. For example:

     \def\eqconstruct#1{\the\chapternumber.#1}

(If you are keeping the chapter number in a count register named
'\chapternumber', naturally.)

   The reason for having both '\eqconstruct' and '\eqprint' may not be
immediately apparent. The difference is that '\eqconstruct' affects the
text that cross-reference label is defined to be, while '\eqprint'
affects only what is typeset on the page. The example just below might
help.

   Usually, you want equation labels to refer to equation numbers. But
sometimes you might want a more complicated text. For example, you might
have an equation '(1)', and then have a variation several pages later
which you want to refer to as '(1*)'.

   Therefore, Eplain allows you to give an optional argument (i.e.,
arbitrary text in square brackets) before the cross-reference label to
'\eqdef'. Then, when you refer to the equation, that text is produced.
Here's how to get the example just mentioned:

     $$...\eqdef{a-eq}$$
     ...
     $$...\eqdef[\eqrefn{a-eq}*]{a-eq-var}$$
     In \eqref{a-eq-var}, we expand on \eqref{a-eq}, ...

We use '\eqrefn' in the cross-reference text, not '\eqref', so that
'\eqprint' is called only once.

   As another example, consider the following requirement: we want to
include chapter number in all equation references, and additionally we
want to include the part number when referencing an equation from any
part other than the one where the equation appears. For example,
references to the third equation in chapter 2 of part 1 should be
typeset as '(2.3)' throughout part 1, but as '(I.2.3)' in any other
part. Let's assume we have the current chapter and part numbers in count
registers '\chapnum' and '\partnum', respectively.

   The idea is to have '\eqconstruct' store the part number of the
equation (that is, the part number _at the time of definition_), so that
later '\eqprint' can compare the stored number with the current part
number (that is, the part number _at the time of reference_). The
complicating factor is that internally, the result of '\eqconstruct' is
both expanded and written out to the '.aux' file, _and_ used to typeset
the equation number, so the commands that store the part number should
behave correctly in both situations. This is difficult to achieve with
expandable commands; therefore, to avoid expansion problems, we are
going to use only TeX primitives, which are non-expandable:

     \newcount\eqpartnum

     \def\eqconstruct#1{%
       \global\eqpartnum=\the\partnum\relax
       \number\chapnum.#1%
     }

     \def\eqprint#1{%
       \setbox0=\hbox{#1}%
       (\ifnum\partnum=\eqpartnum \else
          \uppercase\expandafter{\romannumeral\eqpartnum}.%
        \fi
        \box0)%
     }%

In '\eqconstruct', besides constructing the base equation number (e.g.,
'1.2'), we also store the part number of the equation in the count
register '\eqpartnum' ('\the\partnum' is expanded when the equation
number is written to the '.aux' file, so the equation label definition
in the '.aux' file will contain the actual part number). In '\eqprint',
we need to know the equation's part number before we typeset the base
equation number, therefore we first put the argument in a box, thus
causing '\eqpartnum' to be set.